Why does std::is_same fail when trying to process different container types in a function?

Using std::is_same: Why the Function Fails to Process Different Container Types

In an attempt to create a versatile function capable of printing both stacks and queues, a developer encounters a compilation error. Despite utilizing std::is_same to discern the container's type, the function remains non-functional.

The error originates from the fact that both branches of the if-else statement must compile successfully, which is violated in this instance. To address this issue, a modification is implemented using partial specialization and element_accessor template struct:

<code class="cpp">template &lt;typename Cont&gt;
struct element_accessor;

template &lt;typename T&gt;
struct element_accessor&lt;std::stack&lt;T&gt;&gt; {
   const T&amp;amp; operator()(const std::stack&lt;T&gt;&amp;amp; s) const { return s.top(); }
};

template &lt;typename T&gt;
struct element_accessor&lt;std::queue&lt;T&gt;&gt; {
   const T&amp;amp; operator()(const std::queue&lt;T&gt;&amp;amp; q) const { return q.front(); }
};

template&lt;typename Cont&gt;
void print_container(Cont&amp;amp; cont){
   while(!cont.empty()){
      auto elem = element_accessor&lt;Cont&gt;{}(cont);
      std::cout &lt;&lt; elem &lt;&lt; '\n';
      cont.pop();
   }
}

Alternatively, for C 17 and higher, the error can be circumvented using if constexpr rather than partial specialization:

<code class="cpp">template&lt;template&lt;class&gt; typename Cont, typename T&gt;
void print_container(Cont&lt;T&gt;&amp;amp; cont){
   while(!cont.empty()){
      if constexpr (std::is_same_v&lt;Cont&lt;T&gt;, std::stack&lt;T&gt;&gt;) 
         std::cout &lt;&lt; cont.top() &lt;&lt; '\n';
      else if constexpr (std::is_same_v&lt;Cont&lt;T&gt;, std::queue&lt;T&gt;&gt;) 
         std::cout &lt;&lt; cont.front() &lt;&lt; '\n';
      cont.pop();
   }
}</code>

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